feat(gateway): auto-delete slash-command system notices after TTL (#18266)

Adds opt-in auto-deletion for slash-command reply messages like
"New session started!", "Restarting gateway…", "Stopped.", and
YOLO toggles.  After the TTL elapses the gateway calls the adapter's
delete_message; on platforms without a delete API (everything except
Telegram today) the TTL is silently ignored and the message stays.

Requested on Twitter by @charlesmcdowell — tool-call bubbles are useful
real-time, but system notices clutter the thread once the agent finishes.

Implementation:

- EphemeralReply(str) sentinel in gateway/platforms/base.py.  Subclasses
  str so existing 'X' in response / response.startswith(...) checks in
  tests and call sites keep working unchanged; isinstance() still
  distinguishes it for the send path.
- _process_message_background and both busy-session bypass paths
  (in base.py) call _unwrap_ephemeral() on the handler return, send
  the unwrapped text, and schedule a detached delete task when the
  TTL > 0 AND the adapter class overrides delete_message.
- display.ephemeral_system_ttl (default 0 = disabled) in DEFAULT_CONFIG.
  Handler can pass ttl_seconds explicitly to override.
- Wrapped the highest-noise return sites: /new, /reset, /stop,
  /yolo on/off, /restart success + "already in progress".  Draining
  notices and /help output left as plain strings — those are
  informational and users want to read them.

Backward-compat: default TTL 0 → no scheduling, no behavior change
for existing users.  Platforms without delete_message silently no-op.

hermes_cli/config.py

@@ -775,6 +775,14 @@ DEFAULT_CONFIG = {
         "tool_progress_command": False,  # Enable /verbose command in messaging gateway
         "tool_progress_overrides": {},  # DEPRECATED — use display.platforms instead
         "tool_preview_length": 0,  # Max chars for tool call previews (0 = no limit, show full paths/commands)
+        # Auto-delete system-notice replies (e.g. "✨ New session started!",
+        # "♻ Restarting gateway…", "⚡ Stopped…") after N seconds on platforms
+        # that support message deletion (currently Telegram; other platforms
+        # ignore and leave the message in place).  Only affects slash-command
+        # replies wrapped with gateway.platforms.base.EphemeralReply — agent
+        # responses and content messages are never touched.  Default 0
+        # (disabled) preserves prior behavior.
+        "ephemeral_system_ttl": 0,
         "platforms": {},  # Per-platform display overrides: {"telegram": {"tool_progress": "all"}, "slack": {"tool_progress": "off"}}
         # Gateway runtime-metadata footer appended to the FINAL message of a turn
         # (disabled by default to keep replies minimal). When enabled, renders